Preparation of 1,1,4,4 - tetramethoxy-2-butene

ABSTRACT

A process for the preparation of 1,1,4,4-tetramethoxy-2-butene by reacting 2,5-dimethoxydihydrofuran with methanol in the presence of acids comprises carrying out the reaction in the presence of solid catalysts having acidic centers.

[0001] The present invention relates to an improved process for the preparation of tetramethoxybutene by reacting 2,5-dimethoxydihydrofuran with methanol in the presence of solid catalysts having acidic centers.

[0002] Tetramethoxybutene is an important intermediate for preparing C₁₀-dialdehyde of the formula

[0003] which in turn is a key building block for the synthesis of carotenoids such as β-carotene, astaxanthin and lycopene.

[0004] According to the process as described by C. M. Cox, D. A. Whiting, J. Chem. Soc. Perkin Trans. 1 1991, 1907-1911, V. M. Likhosherstov, Russ. J. Org. Chem. 1983, 19, 1176-1178, and S. M. Makin, N. I. Telefina, Zh. Obshch. Khim. 1962, 32, 1104-1109, and in U.S. Pat. No. 2,768,976 and DE 956 946, tetramethoxybutene is prepared from furan and bromine in methanol. Dimethoxydihydrofuran is formed in situ after 1,4-addition of bromine to the furan and subsequent nucleophilic substitution of the bromine by methanol. Owing to the formation of Br during the bromine substitution, the dimethoxydihydrofuran immediately reacts further to give tetramethoxybutene.

[0005] This process has serious disadvantages: the synthesis has to be conducted at temperatures of from −30 to −50° C., which is difficult to realize technically: the use of bromine requires a high expenditure on safety. Furthermore, bromine is an expensive reagent and highly corrosive, and it is necessary to equip the production plant with expensive specialty materials. In addition, the hydrogen bromide which is formed in equimolar amounts has to be neutralized during work up, producing large amounts of waste salts. It is in principle possible to conduct this synthesis using less expensive chlorine, but the disadvantages described for bromine remain, and the reaction is also significantly slower.

[0006] Since dimethoxydihydrofuran can advantageously be obtained by oxidation of furan in methanol, it has also been described to convert dimethoxydihydrofuran into tetramethoxybutene in the presence of a strong dissolved acid, e.g. p-toluenesulfonic acid or hydrogen chloride. Since this reaction which corresponds to the following reaction equation

[0007] is a particular type of a transacetalization with formation of one mole of water, the water which is released must be removed from the equilibrium. According to the process of EP-A 0 581 097, this problem is solved by adding trimethyl orthoformiate, which is relatively expensive. Furthermore, it is not easy to transfer this process to the continuous scale. Here it is also necessary to neutralize the dissolved catalyst by the addition of a base to be able to stop the reaction at its optimum. Finally, N. Clauson-Kaas, J. T. Nielsen, E. Boss, Acta Chem. Scand. 1955, 9, 111-115, describe the use of aprotic Lewis acids, e.g. boron trifluoride, but the tetramethoxybutene yield was only 9% of theory.

[0008] Another problem of the prior art processes is the formation of considerable amounts of the byproduct pentamethoxybutane, reducing the tetramethoxybutene yield.

[0009] It is an object of the present invention to provide a process which allows the technically simple preparation of tetramethoxybutene, in particular in a continuous manner and in good yield, while reducing the formation of pentamethoxybutane.

[0010] We have found that this object is achieved by carrying out the reaction in the presence of solid catalysts having acidic centers. This results not only in high selectivities, in particular at partial conversion, but also in lower amounts of pentamethoxybutane byproduct. Unconverted dimethoxybutene can be returned to the reaction after distillation of the reaction products with removal of the water which has formed.

[0011] More specifically, the novel process relates to the preparation of 1,1,4,4-tetramethoxy-2-butene by reacting 2,5-dimethoxydihydrofuran with methanol in the presence of acids, which comprises carrying out the reaction in the presence of solid catalysts having acidic centers.

[0012] Solid catalysts having acidic centers are in particular acidic organic ion exchangers or inorganic oxidic catalysts which have acidic centers and are selected from the group consisting of zeolites in the H form, acidic mixed oxides and sheet silicates having acidic centers.

[0013] The catalysts to be used according to the invention essentially belong to four groups consisting of

[0014] a) acidic organic ion exchangers which are preferred,

[0015] b) zeolites in the H form,

[0016] c) acidic mixed oxides, and

[0017] d) sheet silicates having acidic centers.

[0018] a) Acidic organic ion exchangers

[0019] Acidic organic ion exchangers are conventional, partially crosslinked chain polymers which are derived from styrene/divinylbenzene, in particular, and which contain preferably sulfonic acid groups, e.g. of the formula

[0020] Ion exchangers of this type are commercially available, e.g. from DOW Chemical or BAYER Aktiengesellschaft.

[0021] Examples are Dowex® 50WX, Serdolit Red®, Amberlyst® 15, Lewatit® K2431, Navion® H⁺, Amberlite® IR120, Duolite® C20, Lewatit® S100 and Lewatit® K2641.

[0022] b) Zeolites in the H form

[0023] Preference is given to the acidic H form of 12-ring zeolites of the structure type BETA, Y, EMT and mordenite and 10-ring zeolites of the pentasil type. As well as the elements aluminum and silicon, zeolites can also contain boron, gallium, iron or titanium in their framework. Furthermore, they can also be partially exchanged with the elements of group IB, IIB, IIIB, IIIA or VIIIB and the lanthanide elements.

[0024] Zeolites to be used as catalysts include, for example, zeolites in the acidic H form of the structure type MFI, MEL, BOG, BEA, EMT; MOR, FAU, MTW, LTL, NES, CON or MCM-22 according to the structure classification given in W. M. Meier, D. H. Olson, Ch. Baerlocher, Atlas of zeolite Structure Types, Elsevier, 4^(th) ed., 1996.

[0025] Particular examples are the zeolites ZBM-20, Fe-H-ZSM5, Sn-beta zeolite, beta zeolite, Zr-beta zeolite, H-beta zeolite, H-mordenite, USY, Ce-V zeolite, H-Y zeolite, Ti/B-beta zeolite, B-beta zeolite or ZB-10.

[0026] c) Acidic mixed oxides

[0027] The acidic mixed oxides to be used according to the invention are in particular superacidic mixed oxides which have been described repeatedly in the literature. Reference may be made, for example, to R. J. Gillespie, Acc. Chem. Res. 1 (1968) 202 and R. J. Gillespie and T. E. Peel, Adv. Phys. Org. Chem. 9 (972) 1.

[0028] Specific superacidic metal oxides which can be used in the reaction of the invention are disclosed by Kazushi Arata in Applied Catalysis A: General 146 (1996) 3-32 for the reaction of butene and pentanes.

[0029] This reference is incorporated herein for the superacidic metal oxides and their preparation.

[0030] Suitable sulfatized or phosphatized metal oxides (i) are in particular phosphatized or sulfatized zirconium oxide or titanium oxide which may include further elements such as iron, cobalt or manganese.

[0031] Preferred sulfatized or phosphatized catalysts are:

[0032] ZrO₂SO₄ (S content 0.5-4 mol %)

[0033] ZrO₂P₂O₅ (P₂O₅ content 3-20 mol %)

[0034] Fe₂O₃P₂O₅ (P₂O₅ content 3-20 mol %)

[0035] Co/Mn/ZrO₂SO₄ (S content 0.5-4 mol %;

[0036] Co/Mn content 0.1-5 mol %)

[0037] Fe/Mn/ZrO₂SO₄ (S content 0.5-4 mol %;

[0038] Fe/Mn content 0.1-5 mol %)

[0039] Preference is given to superacidic mixed metal oxides of groups (i) and (ii) which contain zirconium, titanium, iron, tin or Cr(III) on the one hand and tungsten or molybdenum on the other.

[0040] Specific examples are TiO₂WO₃, Fe₂O₃WO₃, ZrO₂MoO₃, ZrO₂WO₃, Cr₂O₃WO₃, WO₃TiO₂, TiO₂WO₃ or SnO₂WO₃SiO₂, the molar ratio of the oxides of group (i) to group (ii) usually being from 70:30 to 90:10.

[0041] d) Sheet silicates having acidic centers

[0042] For the purposes of the invention, sheet silicates having acidic centers are those having Lewis and/or Brönsted centers. Therefore, they may be sheet silicates to which said Lewis acids have been applied or which have been treated with acids such as sulfuric acid. However, preference is given to sheet silicates which have negative layer charges neutralized by protons. In this case, the acidic centers of the sheet silicates are essentially Brönsted centers formed in the sheet silicates having excess negative charges by exchange of the metal ions for protons.

[0043] The sheet silicates to be used according to the invention are especially aluminum silicates; they belong to the clay minerals and are composed of SiO₂ tetrahedron and Al₂O₃ octahedron layers, part of the silicon in the tetrahedron layer being replaced by trivalent cations, preferably aluminum, and/or part of the aluminum in the octahedron layer being replaced by bivalent cations, preferably magnesium, so that negative layer charges result.

[0044] Sheet silicates having negative charges occur naturally as montmorillonites, vermiculites or hectorites or can be prepared synthetically.

[0045] A more detailed description is given in Z. M. Thomas and W. Z. Thomas, Principles and Practice of Heterogeneous-Catalysis, 1997, Vetc. ISBN 3-527-29239-8, p. 347 ff.

[0046] However, preference is given to naturally occurring montmorillonite which is converted into its H form by treatment with acids.

[0047] An example is montmorillonite of the formula

Na_(0,33){(Al_(1,67)Mg_(0,33)) (OH)₂[Si₄O₁₀]}

[0048] having layer charges from about 0.6 to 0.2 per formula unit.

[0049] To partially or completely neutralize the negative layer charges with protons, the exchangeable cations, usually alkali metal or alkaline earth metal ions, in the naturally occurring or synthetic sheet silicates are exchanged for protons. This is done in a conventional manner, e.g. by treatment with sulfuric acid or hydrochloric acid.

[0050] Since the sheet silicates containing protons instead of alkali metal or alkaline earth metal ions are thermally less stable, it is also possible to use pillared clays in which the layers are supported against one another. The preparation of such pillared clays is described in detail in Figuras, Catal. Rev. Sci. Eng. 30 (1988) 457 or Jones, Catal. Today (2 (1988) 357, which are incorporated here in the reference.

[0051] Specific examples of sheet silicates having negative layer charges which are neutralized by protons are: montmorillonite, vermiculite and hectorite.

[0052] The catalysts to be used according to the invention are employed in pulverulent or preferably particulate form, for example in the form of granules, extrudates or spheres.

[0053] The heterogeneous catalyst maintains its activity over a prolonged period of time. The inorganic catalysts can then be reactivated, for example by burning off in air at above 450° C. For these reasons the novel process is economically and environmentally particularly advantageous.

[0054] The reaction is either carried out batchwise, for example as a suspension process, or preferably over a fixed-bed catalyst, in a continuous flow reactor.

[0055] In the batchwise reaction, the catalyst is typically used in amounts of from 1 to 50, preferably from 5 to 30, % by weight, based on dimethoxydihydrofuran. Methanol is generally used in excess over the stoichiometrically required amount, for example in 2 to 40 times the molar amount, preferably in a molar excess of from 2 to 20, in particular from 1.5 to 10.

[0056] However, preference is given to the continuous process for which only a slight excess of methanol is required without substantially reducing the yield. It is therefore possible to carry out the reaction at a molar ratio of dimethoxydihydrofuran to methanol of 1:2-1:4, preferably 1:2.4-1:4.

[0057] The reaction according to the invention usually takes place at from −10 to 100° C., preferably from 0 to 40° C., in particular from 15 to 30° C., and at residence times of from 1 to 30 min, preferably from 10 to 60 min.

[0058] In the batchwise embodiment of the invention, dimethoxydihydrofuran is stirred together with an excess of methanol, e.g. in combination with an acidic ion exchanger, at e.g. 0-25° C. for several hours, the catalyst is filtered off and the reaction mixture is worked up by distillation. Unconverted dimethoxydihydrofuran can be reused.

[0059] However, it is advantageous to carry out the reaction in a continuous manner by passing dimethoxydihydrofuran together with methanol over a fixed-bed acidic catalyst. The reaction is advantageously carried out only to a partial conversion, e.g. less than 80% of theory. The mixture leaving the reactor is worked up by distillation and the unconverted dimethoxydihydrofuran and the dehydrated methanol are returned to the reaction.

[0060] Preferred reactors are tubular reactors in which the acidic catalyst is arranged in one or more beds.

EXAMPLES

[0061] (DMD=2,5-dimethoxydihydrofuran; TMB=

[0062] 1,1,4,4-tetramethoxy-2-butene; PMB=

[0063] 1,1,2,4,4-pentamethoxybutane)

Example 1

[0064] Dimethoxydihydrofuran (DMD 25.2 g, 0.19 mol) was added to a suspension of an acidic ion exchanger (29 g of Dowex® 50WX4; DOW Corp.) in methanol (290 ml) at 0° C. while stirring slowly. After stirring at 0° C. for 14 h, the ion exchanger was filtered off. GC analysis of the methanol solution gave 55.7% of tetramethoxybutene (TMB), 42.0% of DMD and 2.0% of pentamethoxybutane (PMB) byproduct, corresponding to a conversion of 58% and a selectivity of 97%. This reaction was carried out at 10° C. and yielded, after a reaction time of 4 h, 57.3% of TMB, 39.4% of DMD and 2.9% of PMB, as determined by GC. This corresponds to a conversion of 61% and a selectivity of 95%.

Example 2

[0065] DMD (25.2 g, 0.19 mol) was added to a suspension of an acidic ion exchanger (29 g of Dowex® 50WX4; (DOW Corp.) in methanol (290 ml) at 10° C. while stirring slowly. After stirring at 10°0 C. for 5 h, the ion exchanger was filtered off. GC analysis of the methanol solution gave 59.1% of TMB, 36.4% of DMD and 4.0% of PMB byproduct, corresponding to a conversion of 64% and a selectivity of 94%.

Example 3

[0066] DMD (8.70 g, 0.07 mol) was added to a suspension of an acidic ion exchanger (10 g of Lewatit K2641) in methanol (100 ml) at 25° C. while stirring slowly. After stirring at 25° C. for 2 h, the ion exchanger was filtered off. GC analysis of the methanol solution gave 53.2% of TMB, 41.2% of DMD and 4.4% of PMB byproduct, corresponding to a conversion of 59% and a selectivity of 92%.

Example 4

[0067] DMD (2.72 g, 0.02 mol) was added to a suspension of an acidic ion exchanger (10 g of Lewatit K2641) in methanol (100 ml) at 25° C. while stirring slowly. After stirring at 25° C. for 30 min, the ion exchanger was filtered off. GC analysis of the methanol solution gave 27.9% of TMB, 67.4% of DMD and 3.3% of PMB byproduct, corresponding to a conversion of 72% and a selectivity of 95%.

Example 5

[0068] Dimethoxydihydrofuran (DMD, 0.68 g, 0.005 mol) was added to a suspension of an acidic ion exchanger (10 g of Lewatit K2641, Bayer) in methanol (100 ml) at 25° C. while stirring slowly. After stirring at 25° C. for 15 min, the ion exchanger was filtered off. GC analysis of the methanol solution gave 34.6% of TMB, 63.1% of DMD and 1.5% of PMB byproduct, corresponding to a conversion of 65% and a selectivity of 98%.

Example 6

[0069] A solution of dimethoxydihydrofuran (110 g) in methanol (1000 g) was kept at 5° C. and pumped from below into a vertical glass column which was filled with Lewatit K2641 ion exchanger. The pump rate was chosen to result in a contact time of 10 h. The glass column was also kept at 0° C. The methanol solution leaving the top of the glass column was analyzed by GC: 54.8% of TMB, 41.1% of DMD, 3.2% of PMB, corresponding to a conversion of 59% and a selectivity of 94%. A residence time of 15 h yielded 58.8% of TMB, 34.5% of DMD, 5.6% of PMB, corresponding to a conversion of 66% and a selectivity of 91%.

Example 7

[0070] A solution of dimethoxydihydrofuran (110 g) in methanol (1000 g) was kept at 0° C. and pumped from below into a vertical glass column which was filled with Dowex® 50WX4 ion exchanger. The pump rate was chosen to result in a contact time of 3 h. The glass column was also kept at 0° C. The methanol solution leaving the top of the glass column was analyzed by GC: 54.9% of TMB, 40.9% of DMD, 3.7% of PMB, corresponding to a conversion of 60% and a selectivity of 94%.

Example 8

[0071] For comparison: reaction using p-toluenesulfonic acid as catalyst

[0072] p-Toluenesulfonic acid (1.9 g, 0.01 mol) was added to a solution of dimethoxydihydrofuran (37.0 g, 0.29 mol) in methanol (420 ml) at 25° C. and the mixture was stirred for 24 h. GC analysis gave 53.0% of TMB, 40.0% of DMD, 7.0% of PMB, corresponding to a conversion of 60% and a selectivity of 88%. 

We claim:
 1. A process for the preparation of 1,1,4,4-tetramethoxy-2-butene by reacting 2,5-dimethoxydihydrofuran with methanol in the presence of acids, which comprises carrying out the reaction in the presence of solid catalysts having acidic centers.
 2. A process as claimed in claim 1, wherein the reaction is carried out in the presence of acidic organic ion exchangers or inorganic oxidic catalysts which have acidic centers and are selected from the group consisting of zeolites in the H form, acidic mixed oxides and sheet silicates having acidic centers.
 3. A process as claimed in claim 1, wherein ion exchangers having sulfonic acid groups are used.
 4. A process as claimed in claim 2, wherein the H form zeolites are of the structure types MFI, MEL, BOG, BEA, EMT, MOR, FAU, MTW, LTL, NES, CON or MCM
 22. 5. A process as claimed in claim 2, wherein the H form zeolites are 12-ring zeolites of the structure type BETA, Y, EMT or mordenite or 10-ring zeolites of the pentasil type.
 6. A process as claimed in claim 2, wherein the reaction is carried out in the presence of a catalyst consisting of or comprising, as an essential constituent, mixed oxides having acidic centers, where the mixed oxides consist of a combination of (i) oxides of titanium, zirconium, hefnium, tin, iron or Cr (III) on the one hand and (ii) oxides of vanadium, chromium (VI), molybdenum, tungsten or scandium on the other or the mixed oxides are sulfatized or phosphatized oxides of group (i) and the mixed oxides have been calcined at from 459° C. to 800° C. after combining.
 7. A process as claimed in claim 2, wherein the catalysts used are sheet silicates doped with Lewis acids and/or sheet silicates which have negative layer charges neutralized by protons.
 8. A process as claimed in claim 1, wherein said 2,5-dimethoxydihydrofuran is continuously reacted with 2 to 40 times the molar amount of methanol at from −10 to 100° C.
 9. A process as claimed in claim 8, wherein the reaction is carried out to a partial conversion of less than 80% of theory, excess methanol and the water which has formed are removed in a subsequent distillation, and unconverted dimethoxydihydrofuran is returned to the reaction. 